1. Field of the Invention
The present invention relates to an artificial joint to be implanted into a human body after resection of an abnormal part in the case where the abnormal part is found in a bone such as the hip joint or the knee joint of the human body.
2. Description of the Related Art
For example, as to the hip joint, as shown in FIG. 4, a total hip replacement type prosthesis comprises a stem 1 and a socket 16, the stem 1 having a head 10 at the upper end thereof. When this prosthesis is implanted into the body of a patient, the stem 1 is embedded in intramedulla of femur 14 side, while the socket 16 on a pelvis 17 side. In many cases, the socket 16 comprises an outer cup (metal back) 2 which comes into contact with a living body bone and an inner cup 11 formed of a high density polyethylene and serving as a rotative slide surface with respect to the head 10. The outer cup 2 embedded in the pelvis 17 is fixed to the acetabulum with screws 12. As long as there does not occur any trouble, the stem and the socket remain embedded in the living body for a long period or during existence of the patient. For their replacement, a large-scale surgical operation is needed. Therefore, it is desirable that there occur no trouble after an initial operation.
According to the present stage of technical level, however, a limit is encountered in the duration of normal function of an artificial joint because it is the artificial joint that is embedded in the living body. There are a very large number of cases where even if the artificial joint exhibits its normal function just after the operation for embedding thereof, a trouble occurs with the lapse of 15 to 20 years. Of course, there are not a few cases where a trouble occurs in a shorter period due to some trouble of the artificial joint itself or due to a poor operation technique.
One serious trouble with the artificial hip joint or knee joint is loosening. This is a phenomenon such that the connection between a prosthesis and a living body bone becomes loose upon lapse of a certain period after a surgical operation. Once this phenomenon occurs, the patient will suffer pain, and there will occur wobbling of the joint to the extent that the artificial joint can no longer stand use.
For preventing the occurrence of such loosening over as long a period as possible after the surgical operation, it is important to improve the operation technique and make an appropriate design of the artificial joint itself. Although various improvements have been made and ample care exercised on the design of the artificial joint, it is the actual situation that the loosening phenomenon often occurs in a shorter period than expected. Therefore, the development of prostheses, including an artificial hip joint, of a superior design is desired.
As an example, reference will be made below to the case where bone cement is not used in an operation.
For preventing the occurrence of loosening over a long period, it is necessary to satisfy many conditions. Although various countermeasures have been proposed, there still remain many problems.
According to a stem of a type without bone cement, as shown in FIGS. 5(a), 5(b) and 5(c), the surface of an upper portion of the stem, indicated at 3, is made porous in order to improve the bonding between a bone of a living body and the stem. This porous portion, indicated at 3, is slightly higher than the base surface which is not porous to improve its contact with the living body bone. Also, the whole surface of an outer cup is made porous, as shown in FIGS. 6(a) and 6(b).
When these stem and outer cup are implanted into a living body, they are each driven forcibly into a slightly tight hole for ensuring a stronger contact with the associated bone. At this time, the particles in the porous surface may drop out partially. Such dropped-out particles staying within the bone can cause bone resorption or loosening.
On the other hand, when the patient's weight is imposed on the artificial hip joint after the surgical operation, the stem becomes easier to rotate, and this is presumed to be a cause of loosening. Therefore, it is desired to design an appropriate stem shape, taking into account by which portion of the stem the patient's weight is to be borne and the rotation is to be prevented and which portions of the stem are to resist varus or valgus deformity and bending.